Tales from the front line in the hunt for the perfect SCART television for use with the MAME arcade game emulator.

Friday, 12 April 2013

How to make VGA to SCART cable (UNFINISHED GUIDE!)

This is a step-by-step guide that I started to make a long time ago. I've always intended to redo this guide (there are some mistakes in there along the way) but have now come to the conclusion that I'll probably never find the time.

I'm posting this guide in draft form in case there are some construction ideas in here that help others.

Also, here are some useful diagrams for anyone wanting to build their own VGA to SCART cable:

Circuit diagram

Veroboard layout

Here's where the draft guide starts:

This guide will show you how to make a VGA to SCART cable that is compatible with both ATI and also Nvidia cards.

There are many VGA to SCART schematics to be found on the Internet.

The purpose of this guide is provide a step-by-step set of instructions that use photos instead of diagrams to show how the cable is constructed.

Tidy the rough end of the VGA cable (Fig. 1) by cutting a clean, straight line using a knife (Fig. 1a).

Fig. 1a – Rough end

Then, mark a line 50 mm from the end of the cable using a pen (Fig. 2).

Fig. 2 - 50 mm mark

Insert the plastic nut for SCART plug onto the onto the cable. It's very important to do this now before beginning to dress the VGA cable as it will be impossible to place this on after soldering begins. Next, cut a piece a 50 mm length of heatshrink that has a diameter large enough to fit over the cable. Choose a size that will also accommodate your audio cable later (10 mm should be sufficient). Slide this onto the cable end (Fig. 3).

Fig. 3

Carefully cut around the circumference of the cable at the 50mm mark. Be gentle so as not to damage the metal shielding underneath the plastic (Fig. 4).

Fig. 4 – Outer insulation removed to expose shielding

Next, carefully unpick the braiding using a pointy tool (Fig. 5). Use a plastic hair comb to straighten out the strands once they are loose.

Fig. 5 – Braid separated

Now, cut off the excess strands and twist the remainder into one strand that is around 0.5 mm in diameter (Fig. 6a). Tin the wire 10mm from the end (Fig. 6b)

Fig. 6a – Twisted braid wire

Unfold the foil to expose the wires beneath and cut the foil away (Fig. 7).

Fig. 7 – Foil removed

The next steps are more complicated. We need to find out which wires relate to which pins on the VGA connector.

First, identify the wires which look like the R, G and B lines. These will have shield wire wrapped around them (as in Fig. 7) or folded foil. Ignore these wires for the moment. Strip approximately 3 mm of insulation off the end of non-RGB wires. Then twist the ends of each to keep them neat (Fig. 8).

Fig. 8

Next, we need to use a digital multi-meter (DMM) to match the wire colours to the VGA pins. Setting up the VGA plug and exposed wires in a pair of vices helps greatly in this task (Fig. 9).

Fig. 9

Check the pin numbering inside the VGA plug (Fig. 10) before you start checking and making notes (Fig. 11).

Now, cut out the any wires that are unneeded leaving only RGB lines and the 4 wires we need. Note: the colours in your cable will most likely be different to those shown Fig. 9 and listed in Fig. 11. Tin the remaining wires using your soldering iron (Fig. 12)

Fig. 12

Next, we need to prepare the R, G, and B conductors and corresponding ground wires. Unravel the shield for the red wire. Twist the shield together into one strand about 0.5 mm thick. Trim some of the excess shielding if before twisting if you think the combined result will be too thick. Next, tin about 10 mm of the twisted shield. Cut a piece of heat shrink tubing (preferably red) to a 45 mm length. Slide it over the twisted wire and shrink it. Next, strip about 3 mm of the red conductor wire and then tin the end (Fig. 13).

Fig. 13

Now, repeat the previous steps for the green and blue conductors and matching shields. Once done, apply 45 mm of heatshrink to to the shield wire we prepared in Fig. 6. Now, if all steps were followed correctly, we have prepared 11 wires in total (Fig. 14).

Fig. 14

The VGA cable is now complete!

Preparing the sync circuit:

Cut a piece of Veroboard to size. It will need to be 7 across and 9 down (Fig. 1a). Make sure you have orientation of the conductive strips correct (Fig. 1b).

Fig. 1a

Fig. 1b

Now, we need to cut the board down further in order to make it easy to fit inside the plastic SCART hood.

Fig. 2a

Fig. 2b

Fig. 2c

Now we're ready to start soldering in the components...

Solder in the 3.3k resistor:

Fig. 3a - 3.3k resistor

Fig. 3b

Fig. 3c - Top view

Solder in the 68 ohm resistor:

Fig. 4a - 68 ohm resistor

Fig. 4b

Fig. 4c - Top view

Solder in the 1.2k resistor:

Fig. 5a - 1.2k resistor

Fig. 5b

Fig. 5c - Top view

Solder in the 820 ohm resistor:

Fig. 6a - 820 ohm resistor

Fig. 6b

Fig. 6c - Top view

Solder in the BC548 transistor:

Fig. 7a - BC548 transistor

Fig. 7b

Fig. 7c - Top view

Next, cut two pieces of wire to a 50mm length each. Trim the ends to about 2mm and tin (Fig. 8a). Then, solder these wires to the sync circuit (Fig. 8b).

Mount the VGA cable in a vice and face it towards the SCART plug. Place the heatshrink on the RGB wires (Fig. 3).

Fig. 3

Now, solder the R, G and B lines to the matching solder cups (Fig. 4) Double check the connections between the VGA and SCART plugs using a multimeter. It's much harder to fixed mismatched RGB wires beyond this point.

Fig. 4

Apply the heatshrink to the RGB SCART pins (Fig. 5).

Fig. 5 - RGB heatshrink applied

Flip the SCART plug in the vice. Fill the solder cups according to Fig. 6.

Fig. 6

Solder a 68 ohm resistor as shown (Fig. 7a). Cut a 15 mm piece of 2.5 mm diameter heat shrink. Slide it over the +5v wire from the VGA cable (refer back to Fig. 11 in the VGA section). Solder the +5v wire to the resistor and then heat shrink with a 15mm length of tube (Fig. 7b).

Well I can't tell you how it will make the dreamcast look but I can tell you definitively that VGA cabling offers clearer and higher resolution output than a SCART, RGB, coax, composite, or component cable will. I use VGA cables on my CRT monitors to display 2 20'' monitors at 1900x1440 each. It may make it look clearer on your HDTV set but it will never look extremely crisp as it is always being stretched to fit that TV. The dreamcast only outputs in 480 so you will never make it higher resolution that that. If it functions anything like my component cable on my ps2 then it will crispen the image on the HDTV. There is a massive difference between composite and component. You could always give it a go and just take the cables back if they dont work. many local electronic stores have HDMI to VGA Cable for very cheap.